1. Field of the Invention
Aspects of the present invention relate to a near field optical recording/reproducing apparatus, and a method of normalizing a gap error signal of the same, and more particularly, to a near field optical recording/reproducing apparatus which can maintain a gap error signal constant during recording and reproducing operations by normalizing the gap error signal, and a method of normalizing the gap error signal of the same.
2. Description of the Related Art
Technology for storing information on an optical recording medium, such as an optical disc, has matured and BDs (Blu-ray discs), HD-DVDs (High Definition DVDs), as well as CDs (compact discs) and DVDs (digital versatile discs) are now commercialized. Such commercialized technologies for CDs, DVDs, BDs, and HD-DVDs involve far field storage technology in which a distance from an optical head of an optical recording/reproducing apparatus to a recording surface of an optical disc is about 1-2 mm. For such far field storage technology, even when the wavelength of light is decreased and the numerical aperture (NA) of an objective lens is increased, decreasing the size of an optical spot formed on the recording surface of the optical disc is limited due to a light diffraction phenomenon which makes it difficult to increase a recording capacity of the optical disc.
Recently, attention has been focused on an optical recording/reproducing apparatus using near field storage technology since such an optical recording/reproducing apparatus can provide a large recording capacity of over a terabyte per optical disc. For near field storage technology, by reducing a distance from an optical head thereof to a recording surface of an optical disc to be not greater than 100 nm, recording and reproducing operations are performed by transferring to (or shining on) an optical disc, an evanescent wave of light generated in a very narrow area corresponding to several tenths of the light wavelength. For example, for near field optical recording/reproducing technology using a solid immersion lens (SIL), when an optical spot is formed on a surface of the SIL by converging an incident beam with an objective lens, a considerable portion of the optical spot is totally reflected by the SIL and proceeds back to the objective lens. At this time, light of a very slight intensity exists on an opposite surface of the SIL and such a light is referred to as an evanescent wave. Thus, information can be recorded or reproduced with respect to the optical disc by transferring the energy of the evanescent wave to the optical disc through a narrow air gap between the SIL and the optical disc.
To optimize the recording and reproduction quality using the near field storage technology, it is important to maintain a constant distance between the SIL and the optical disc. Typically, the distance between the SIL and the optical disc can be obtained by measuring the intensity of light that is being totally reflected by the SIL. That is, the facts that an amount of light totally reflected by the SIL is maximized in a far field state, and an amount of light that is totally reflected by the SIL decreases as the distance between the SIL and the optical disc decreases in a near field state, are used. A light signal that proceeds back by being totally reflected by the SIL is generally referred to as a gap error signal (GES). Thus, the distance between the SIL and the optical disc can be estimated from an amplitude of the GES. For example, assuming that a voltage level of the GES in the far field state is 1 V, and the voltage level of the GES when the SIL completely contacts the optical disc is 0 V, then, when the voltage level of the GES is 0.5 V, the distance between the SIL and the optical disc can be estimated to be about 50 nm, for example.
However, since an amplitude of the GES is affected by the intensity of the light provided by a light source, when an intensity of a driving signal for driving the light source changes, it is difficult to accurately measure or detect the distance between the SIL and the optical disc. In particular, since the intensity of the light during a recording operation is about ten times greater than that during a reproduction operation, the amplitude of the GES remarkably changes when a recording mode is switched to a reproducing mode or vice versa. Thus, in order to maintain a constant distance between the SIL and the optical disc as a recording mode is switched to a reproducing mode or vice versa, normalization of the GES is needed to make the GES constant regardless of the intensity of the driving signal of the light source.